1. Field of the Invention
The present invention relates to a photoelectric switch, and in particular, to a photoelectric switch including a light projecting circuit which can be changed over to increase an amount of projected light in occasions such as in adjustment of an optical axis and further to a photoelectric switch associated with optical fibers in which a projected light is introduced to a sense position by means of an optical fiber and a transmitted light or a reflection light thereof is delivered from the sense position to a light receiving element by use of an optical fiber.
2. Description of the Prior Art
Heretofore, in a light projecting circuit of an photoelectric switch, a light-emitting diode (LED) is used as a light projecting element. The LED is connected to a resistor for limiting a current flowing therethrough and a transistor as a driver thereof. The transistor has a base, which is supplied with a pulse signal from an oscillator so as to be turned on and off in response thereto, thereby resultantly blinking the LED by use of the pulse signal. As shown in FIG. 8, the photoelectric switch of this type can be used in a wide range of environmental temperatures T ranging from -25.degree. C. to 55.degree. C. The LED is assigned with a rated current I.sub.B, which is decreased as the temperature T is increased. In consequence, the driving current of the light projecting LED has a limit associated with a rated current developed at an upper limit of the environmental temperature (I.sub.A of FIG. 8).
In the conventional photoelectric switch, the rated current obtained at the upper limit of the environmental temperature limits the light projecting current. Consequently, the amount of the light thus projected from the LED is not sufficient. Even when a red-light LED is employed, a position of a beam of lights is not easily determined and hence there has been a problem that a considerably long period of time is necessary to align a light axis, for example, in a light receiving section.
On the other hand, in a photoelectric switch employing optical fibers in which the fibers connect a sense position to a sensor unit, for example, as shown in FIG. 14, a light projecting element 6 of a sensor unit 101 is intermittently driven (with pulsated signals or pulses) by means of an oscillator circuit 103 so as to deliver a light emitting output from the light emitting element 6 to a light projecting optical fiber 7. Disposed to oppose an end surface of the optical fiber 7 is an end surface a light receiving optical fiber 8. Through the optical fiber 8, a light signal is delivered so as to be sensed by a light receiving element 9, which converts the light signal into an electric signal to be processed through an amplifier circuit 106, a detection circuit 107, and a waveform shaping circuit 108. The electric signal is thus converted into a rectangular wave to be outputted as an object sense signal via an output circuit 109 and to an indicator lamp 110, which thus indicates a sense state of an object. In the conventional photoelectric switch using optical fibers, since the optical fiber has a small diameter and is flexible, such a photoelectric switch is arranged in a position where the ordinary photoelectric switch not employing the optical fiber cannot be easily installed. In this arrangement, the optical fibers 7 and 8 are extended from the sensor unit 101. With the provision of this configuration in which an optical system of the photoelectric switch is constituted with optical fibers having a small diameter, it is possible to sense a fine or very small object.
However, in the photoelectric switch adopting the optical fibers as described above, since the end portions of the optical fibers 7 and 8 are separated from the sensor unit, in an alignment of optical axes of the optical fibers and in a position alignment to adjust a distance to an object, it is necessary to confirm the blinking state of the indicator lamp 110 of the sensor unit 101 disposed at a location separated from the end portions of the optical fibers 7 and 8. Consequently, when the sensing position (associated with the end positions of the optical fibers) is apart from a location of the sensor unit or when a fine adjustment is required in the setting of the apparatus, there has been a disadvantage that the adjustment takes a long period of time.